The present disclosure relates to a sheet supply device for supplying a sheet like a paper, and an image forming apparatus.
The sheet supply device is widely applied to the image forming apparatus like the copying machine, the facsimile machine, the scanner, and the multifunction apparatus. Using the sheet supply device, the sheets like papers to transfer an original image thereon are conveyed one by one to a position to form the image thereon, whereby the image forming apparatus can print each image successively.
The sheet supply device is configured so that a sheet bundle abuts on a pickup roller in order that a plurality of sheets stacked onto a tray is fed one by one. A thickness of sheet bundle depends on the number of sheets stacked on the tray. Accordingly, a distance between a sheet stacking surface and the pickup roller should be adjusted according to the thickness of sheet bundle, in order to allow the sheet bundle to abut on the pickup roller properly. A lifting plate liftably mounted on the sheet stacking surface can be used for such adjustment. In the sheet supply device, the sheet bundle on the lifting plate is pushed up by the lifting plate, and the sheet bundle abuts on the pickup roller. The pickup roller sends out the sheet abutting on the pickup roller.
There are various kinds of techniques for materializing a stable sheet feeding in such sheet supply device. For instance, one configuration is disclosed wherein a torsion spring is employed as a lifting member for lifting up the lifting plate, and the elastic deformation of the torsion spring is changed depending on the paper remaining amount so that the force pushing up the lifting plate can be varied. In this configuration, it is possible to generate an almost constant feeding pressure (the force pushing sheet bundle against the pickup roller) regardless of the change of the loading weight from the full loading to the last sheet.
In addition, another configuration is disclosed wherein a plurality of elastic members is employed as the lifting member for lifting up the lifting plate. In such configuration, the force for lifting up the lifting plate can be varied by using an elastic member for generating a linear elastic force and the other elastic member for generating a non-linear elastic force.
Moreover, there is another configuration to suppress the conveyance skid by increasing the feeding pressure when the time for the fed sheet to arrive at the position to detect the paper arrival is longer than the specific time.
The image forming apparatus such as the printers has been requested to improve the image forming speed (the printing speed). On that account, since the sheet supply device is also needed to improve the paper feeding speed, the rotating speed (peripheral speed) of the pickup roller is increased. Where the rotating speed of the pickup roller is increased, the frictional force, which is generated between the pickup roller and the uppermost paper at the start of the pickup roller rotation, also tends to increase.
Moreover, there is a request of price reduction of the image forming apparatus, and it is considered that the cost of members composing the image forming apparatus is cut down. As the measure to down the cost, it is considered that the member made of metal is replaced with the member made of resin such as plastic. For instance, regarding the sheet supply device, it is considered that the member made of resin is used as the lifting plate uses.
When the resin member is applied to the lifting plate of the sheet supply device wherein the peripheral speed of the pickup roller is large as described above, however, the lifting plate is warped by the frictional force. When the warp is generated on the lifting plate in such way, the force for pushing the sheet bundle to the pickup roller is reduced. As a result, the paper feeding pressure is reduced, and the sheet cannot be sent out appropriately. That is to say, an interval between the drive start time of the pickup roller and the finishing time of the sheet feeding gets long. When it is determined that the lifting plate is to be lifted up because the warp occurs on the lifting plate and the force for pushing the sheet bundle to the pickup roller is reduced, the time is required for lifting the lifting plate, too. In this case, the interval between the drive start time of the pickup roller and the finishing time of the sheet feeding become longer than ever.
In the sheet supply device, in order to determine whether or not the sheet is conveyed normally, a sensor is disposed on the downstream side of the pickup roller, and the interval between the drive start time of the pickup roller and the detection time that the sensor detects the sheet fed by the pickup roller is measured. When the measured time is longer than a predetermined maximum time, it is determined the abnormal of sheet feeding (JAM). In the above-mentioned high-speed sheet supply device, there is a tendency that the maximum time is set to be short. Therefore, when the sheet feeding time gets long due to the warp of the lifting plate, the abnormality of sheet feeding is detected frequently.
Since the above-mentioned sheet conveyance failure is caused by the warp of the lifting sheet, even if the method for keeping the feeding pressure constant and the method for increasing the feeding pressure, as disclosed in the foregoing conventional arts, are applied to the sheet supply device, it is not possible to eliminate the sheet failure.